31 Euphrosyne


Euphrosyne is perhaps the 12th-largest asteroid in the asteroid belt, and possibly one of the half-dozen or so most massive. It was discovered by James Ferguson on September 1, 1854, the first asteroid found from North America. It is named after Euphrosyne, one of the Charites in Greek mythology. In 2019 a small companion was discovered.

Observations

Euphrosyne is a fairly dark body near the belt's outer edge. Consequently, Euphrosyne is never visible with binoculars, having a maximum apparent magnitude at the best possible opposition of around +10.2, which is fainter than any of the thirty asteroids previously discovered.
Euphrosyne has a high orbital inclination and eccentricity having nodes near perihelion and aphelion, Euphrosyne's perihelion lies at the northernmost point of its orbit. During perihelic oppositions, Euphrosyne is very high in the sky from northern latitudes and invisible from southern countries such as New Zealand and Chile.
Euphrosyne is very little studied but has been targeted by radar.
It is a nearly-spherical C-type asteroid with a primitive surface possibly covered by thick ejection blanket in same collision which created its collisional family. Its rotation period is typical for large asteroids. Nothing is known of its axial tilt.

Mass

The high mass estimate calculated by Baer et al.,, would make Euphrosyne the 5th- to 8th-most-massive asteroid, after the "big four" and within a standard deviation of 704 Interamnia, 511 Davida and 532 Herculina. However, the authors noted that their calculation – which was based on the perturbation of a single other asteroid – was suspect, as the high resulting density was inconsistent with a C-type asteroid. Nonetheless, the preliminary orbit later reported for the satellite is consistent with such a high mass.
The measurements of orbit of satellite have allowed a more precise mass estimation of 31 Euphrosyne in 2020. The improved mass estimation is 1.7*1019 kg.

Satellite and family

Euphrosyne is the namesake of a complex family two thousand asteroids that share similar spectral properties and orbital elements. They are thought to have arisen from a collision. All members have relatively high orbital inclinations.
In 2019 a small satellite was discovered, likely resulting from the same collisional event that created the family. Preliminary orbit computations indicated an orbital period of approximately 1.2 days and a semi-major axis of 677 km. The difference in absolute magnitude is 8.0 ± 0.8, implying a size ratio of and thus a diameter of.